*2.1. Physicochemical Structural Characterization of TO@HNT Hybrid Nanostructure*

The obtained TO@HNT hybrid nanostructure as well as the pure HNT were characterized with TG experiments, FTIR spectra, XRD analysis, and DSC measurements. TG, FTIR, XRD, and DSC plots of pure HNT and TO@HNT hybrid are shown in Figure 1a–d respectively. TGA plots of both materials in Figure 1a indicate that in both cases exist two mass loss steps. The first one starts at around 50 ◦C and ends at around 400 ◦C. The second mass loss step which is attributed to the HNT dehydration process starts at around 450 ◦C and ends at around 600–700 ◦C [44–46]. In the case of pure HNT, the first mass loss step represents the mass loss of superficially adsorbed water and the second step represents the

mass loss of structural/trapped water. In the case of the TO@HNT hybrid, the first step represents the loss of both water and TO molecules. Hence, by subtracting the mass lost from the first mass loss step of pure HNT from the mass lost from the first mass loss step of the TO@HNT hybrid, we calculated an average TO loading on HNT equal to 34.5 wt.%. the mass lost from the first mass loss step of pure HNT from the mass lost from the first mass loss step of the TO@HNT hybrid, we calculated an average TO loading on HNT equal to 34.5 wt.%.

*2.1. Physicochemical Structural Characterization of TO@HNT Hybrid Nanostructure*

The obtained TO@HNT hybrid nanostructure as well as the pure HNT were characterized with TG experiments, FTIR spectra, XRD analysis, and DSC measurements. TG, FTIR, XRD, and DSC plots of pure HNT and TO@HNT hybrid are shown in Figure 1a–d respectively. TGA plots of both materials in Figure 1a indicate that in both cases exist two mass loss steps. The first one starts at around 50 °C and ends at around 400 °C. The second mass loss step which is attributed to the HNT dehydration process starts at around 450 °C and ends at around 600*–*700 °C [44–46]. In the case of pure HNT, the first mass loss step represents the mass loss of superficially adsorbed water and the second step represents the mass loss of structural/trapped water. In the case of the TO@HNT hybrid, the first step represents the loss of both water and TO molecules. Hence, by subtracting

*Gels* **2022**, *8*, x FOR PEER REVIEW 3 of 25

**Figure 1.** (**a**) TG plots of (1) pure HNT and (2) TO@HNT hybrid nanostructure, (**b**) FTIR plots of (1) TO, (2) pure HNT, and (3) TO@HNT hybrid nanostructure, (**c**) XRD plots of (1) pure HNT, and TO@HNT hybrid nanostructure, and (**d**) DSC plots of pure HNT (line 1) and modified TO@HNT (line 2)*.* **Figure 1.** (**a**) TG plots of (1) pure HNT and (2) TO@HNT hybrid nanostructure, (**b**) FTIR plots of (1) TO, (2) pure HNT, and (3) TO@HNT hybrid nanostructure, (**c**) XRD plots of (1) pure HNT, and TO@HNT hybrid nanostructure, and (**d**) DSC plots of pure HNT (line 1) and modified TO@HNT (line 2).

In Figure 1b the FTIR spectra of both pure HNT and modified TO@HNT hybrid nanostructure are plotted. In the FTIR plot of TO, a broad peak in the range of 3530 to 3433 cm−1 was assigned to the stretching vibration of O-H groups [45]. The bands at ~3100–3000 cm*−*<sup>1</sup> are corresponded to aromatic and alkenic -CH=CH- stretch vibrations [36]. The absorption bands in the range 2958 to 2868 cm−1 are assigned to the stretching mode of C-H groups [45]. The bands between 1500 cm−1 and 1300 cm−<sup>1</sup> are assigned to the C-H In Figure 1b the FTIR spectra of both pure HNT and modified TO@HNT hybrid nanostructure are plotted. In the FTIR plot of TO, a broad peak in the range of 3530 to 3433 cm−<sup>1</sup> was assigned to the stretching vibration of O-H groups [45]. The bands at ~3100–3000 cm−<sup>1</sup> are corresponded to aromatic and alkenic -CH=CH- stretch vibrations [36]. The absorption bands in the range 2958 to 2868 cm−<sup>1</sup> are assigned to the stretching mode of C-H groups [45]. The bands between 1500 cm−<sup>1</sup> and 1300 cm−<sup>1</sup> are assigned to the C-H bending of the C-O-H and aliphatic CH<sup>2</sup> groups bending [36].

bending of the C-O-H and aliphatic CH<sup>2</sup> groups bending [36]. In the FTIR plot of pure HNT, the bands at 3700 and 3620 cm−1 are assigned to hydroxyl groups in the internal HNT's surface. The weak band at 3540 cm−1 is assigned to the Si–O–Si (Al) groups. The intense absorption bands in the region of 1100–1000 cm−1 and at 790 cm*−*<sup>1</sup> are assigned to Si–O group. The band at 910 cm*−*<sup>1</sup> is assigned to the hydroxyl groups bending vibration. The band at 745 cm −1 is assigned to the Si–O–Al bonds [44,45]. In the FTIR plot of pure HNT, the bands at 3700 and 3620 cm−<sup>1</sup> are assigned to hydroxyl groups in the internal HNT's surface. The weak band at 3540 cm−<sup>1</sup> is assigned to the Si–O–Si (Al) groups. The intense absorption bands in the region of 1100–1000 cm−<sup>1</sup> and at 790 cm−<sup>1</sup> are assigned to Si–O group. The band at 910 cm−<sup>1</sup> is assigned to the hydroxyl groups bending vibration. The band at 745 cm <sup>−</sup><sup>1</sup> is assigned to the Si–O–Al bonds [44,45]. In the FTIR plot of hybrid TO@HNT are assigned the same bands with pure HNT and additionally the characteristic bands of TO mentioned hereabove. The characteristic bands of TO in the FTIR plot of TO@HNT imply the adsorption of TO on the HNT surface. No shift peak of HNT bands was obtained in the FTIR plot of the TO@HNT hybrid implying rather physisorbed than chemisorbed adsorption of TO on the HNT surface.

In Figure 1c the XRD plot of pure HNT and modified TO@HNT powders are shown. In both XRD plots which were obtained, the halloysite's distinct diffraction peaks at 2θ = 12.0, 20.1, and 24.6 are obvious, and correspond to (001), (100), and (002) planes respectively due to the crystalline property of the HNT [47]. In the case of pure HNT, the presence

of the (001) peak at 2θ of 12.1◦ corresponds to a layer spacing of 0.73 nm. In the case of modified TO@HNT hybrid nanostructure, the peak at 2θ of 11.7◦ corresponds to a layer spacing of 0.76 nm. This difference of approx. 0.03 nm is too small and indicates probably the insertion of small water molecules in HNT's interlayer space. In the case of thymol molecules insertion in the HNT's interlayer space, it should be expected a larger opening of HNT's interlayer space as the thymol molecule size is bigger than that of phenol size (0.4 nm) [36]. So, XRD results indicated that adsorption of thymol took place on the external surface of HNT and no changes in the crystal structure of HNT are obtained due to the TO adsorption process. This result is in accordance with Shemesh et al. [48] where carvacrol a molecule similar to thymol loaded on the external surface of HNT.

In Figure 1d the DSC plots of pure HNT (line (1)) and modified TO@HNT (line (2)), nanohybrid are presented. In the DSC plot of pure HNT, the exothermic peak at 164 ◦C with a ∆H equal to 67.88 J/g is assigned to the desorption process of water molecules. In the DSC plot of TO@HNT nanohybrid, the exothermic peak at 227 ◦C with a ∆H equal to 227.6 J/g is assigned to the desorption of TO molecules in accordance with the previous report [49]. Thus, DSC analysis indicates the absorption of rich TO molecules on the HNT surface.

The overall characterization of TO@HNT nanohybrids concludes that a rich in TO fraction is physiosorbed on the external HNT's surface and validates the distillation/evaporation adsorption process followed.
